Modal Realism and Inductive Scepticism

Modal Realism and Inductive ScepticismAuthor(s): Holly ThomasSource: Noûs, Vol. 27, No. 3 (Sep., 1993), pp. 331-354Published by: WileyStable URL: http://www.jstor.org/stable/2215937 .

Accessed: 15/06/2014 19:16

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

Wiley is collaborating with JSTOR to digitize, preserve and extend access to Noûs.

http://www.jstor.org

This content downloaded from 91.229.229.162 on Sun, 15 Jun 2014 19:16:21 PMAll use subject to JSTOR Terms and Conditions

http://www.jstor.org/action/showPublisher?publisherCode=black

http://www.jstor.org/stable/2215937?origin=JSTOR-pdf

http://www.jstor.org/page/info/about/policies/terms.jsp


NOUS 27:3 (1993) 331-354

Modal Realism and Inductive Scepticism

HOLLY THOMAS

University of California, Santa Cruz

I. Introduction

I wish to argue for an objection to David Lewis's modal realism that goes roughly as follows: Acceptance of Lewis's theory makes radical scepticism with respect to induction unavoidable, and whatever attractions the theory may have, this is too high a price to pay for accepting it. Versions of this objection have been formulated by Peter Forrest, George Schlesinger, Robert Adams, and J. J. C. Smart, among others. I will call it 'the Objection from Scepticism'.

It is possible, of course, to argue that relying on induction is unreasonable, period-not merely for the modal realist, but for anyone. Let us call such an argument a 'Hume Argument' since several instances are to be found in Hume's Treatise and Enquiry. In response to the Objection from Scepticism, Lewis suggests, in effect, that any argument for the conclusion that the modal realist cannot reasonably trust induction must really be a Hume Argument in disguise. If the Objection from Scepticism is really a Hume Argument, then it is illegiti- mate; Hume Arguments tar everyone with the same brush, and so cannot provide the basis for a legitimate objection to anyone's theory in particular.

I will try to formulate the Objection from Scepticism so as to make it quite clear that it is specific to modal realism. But it is worth considering whether the objection might, after all, have a connection with some Hume Arguments. Few, if any, philosophers are willing to embrace scepticism towards induction, but it is often difficult to see what is wrong with a particular Hume Argument. Perhaps what is wrong with some Hume Arguments is that they implicitly incorporate one or more assumptions to which not everyone is committed regardless of his theory of modality. If this is so, then the Objection from Scepticism may be interesting

?D,1993 Basil Blackwell, Inc., 238 Main Street, Cambridge, MA 02142, USA, and 108 Cowley Road, Oxford OX4 1JF, UK.

331



332 NOUS

not merely as one nail in the coffin of Lewis's modal realism; understanding it may reveal some of the less obvious commitments concerning modality that are involved in regarding induction as reasonable.

II. Some Preliminaries: Modal Realism and Induction

Modal realism implies a commitment to the literal existence of entities that are regarded as possible worlds. A modal realist typically takes expressions such as 'It is possible that ...', and 'It is necessary that...' to mean the same, respectively, as 'There is a possible world in which...', and 'In every possible world...'.

There are others besides Lewis who grant the existence of things they call 'possible worlds', and thereby qualify as modal realists. Lewis's theory differs from competing forms of modal realism chiefly in that Lewis takes possible worlds to be concrete entities.1

Adherents of other forms of modal realism typically take the things they call 'possible worlds' to be abstract entities of one sort or another, such as properties (e.g., ways a world could have been), or sets (e.g., of propositions). Lewis, however, claims that the things he calls 'possible worlds' are literally worlds, concrete individuals not different in nature from the actual world. The actual world is not, on his view, distinguished in any essential way from other possible worlds. He takes 'actual' to be an indexical term, like 'here' or 'now', and the actual world simply to be the one in which we happen to live.

Lewis classes all forms of modal realism besides his own as varieties of ersatz modal realism, and his own theory as genuine modal realism. An "ersatz realist" is unlikely to be satisfied with this way of characterizing his view, so I will adopt a more neutral pair of terms. When a distinction needs to be made, I will refer to Lewis's theory as 'concrete modal realism', to other forms of realism as 'abstract modal realism', to Lewis's worlds as 'concrete worlds', and to the worlds of an abstract realist as 'abstract worlds'.

The Objection from Scepticism depends not only on Lewis's views as to the nature of possible worlds, but also on his views as to their number and variety. He takes his theory to require a "principle of plenitude"-a guarantee that there are enough possible worlds to do all of the work he wants them to do. His preliminary attempt to express such a principle is this:

[A]bsolutely every way that a world could possibly be is a way that some world is. [Lewis, 1986, p. 86]2

Ultimately, however, Lewis wants to eliminate problematic abstract entities, such as ways a world could be, in favor of the worlds themselves:

Given modal realism, it becomes advantageous to identify 'ways a world could possibly be' with the worlds themselves. Why distinguish two closely corresponding entities: a world, and also the maximally specific way that world is? Economy dictates identifying the 'ways' with the worlds. [Lewis, 1986, p. 86]



MODAL REALISMAND INDUCTIVE SCEPTICISM 333

The identification of 'ways' with 'worlds' may be economical, but it means that Lewis's preliminary attempt at formulating a principle of plenitude conveys only the trivial information that every possible world is a possible world, as he himself is well aware.3

His problem is to find a substantive principle asserting the existence of "enough" possible worlds, without implying the existence of abstract entities, such as ways a world could be, distinct from the worlds themselves. His solution is to adopt a "Humean denial of necessary connections between distinct exis- tences"-the Recombination Principle-which may be stated as follows:

There is a world in which a duplicate of anything coexists with a duplicate of anything else (size and shape permitting). [I take this to be the gist of the discussion on pp. 89-91 of Lewis, 1986.]

Coexistence is what Lewis elsewhere calls 'the world-mate relation'. Two things coexist when they bear any spatio-temporal relation at all to one another; they don't necessarily have to exist simultaneously. Everything in a given world bears this relation to everything else in that world, and to nothing else.

Lewis explains the notion of duplication in the following passage:

Duplication is a matter of shared properties, but differently situated duplicates do not share all their properties. In section 1.5 [of Lewis, 1986] I defined duplication in terms of the sharing of perfectly natural properties, then defined intrinsic properties as those that never differ between duplicates. That left it open that duplicates might differ extrinsically in their relation to their surroundings. Duplicate molecules in this world might differ in that one is and another isn't part of a cat. Duplicate dragons in different worlds may differ in that one coexists with a unicorn and the other doesn't. Duplicate heads may differ in that one is attached to the rest of a human body and the other isn't. [Lewis, 1986, p. 89]

One may understand the Recombination Principle by imagining that a new possible world could be manufactured by copying pieces of the actual world and then pasting the copies together in new spatio-temporal arrangements. Two different things cannot be pasted into exactly the same place, and there may be a limit to how much can be crammed into a single world, but we are to imagine no other limitations on the worlds that can be manufactured by copying and pasting. His acceptance of the Recombination Principle commits Lewis to the existence of all the worlds we can imagine manufacturing in this way.4

The Objection from Scepticism challenges Lewis's ability, as a modal realist, reasonably to rely on induction, so a few words need to be said about what I mean by 'induction'. I accept Lewis's characterization of 'induction' as desig- nating "all the methods we deem reasonable for forming beliefs about the unobserved parts of our world on the basis of experience with the observed parts." [Lewis, 1986, p. 116]



334 NOUS

III. A Preliminary Formulation of the Objection From Scepticism

Here is a preliminary formulation of the Objection from Scepticism5:

(O1) Acceptance of Lewis's modal realism undermines one's reasonable reliance on induction.

(02) Acceptance of other theories of modality does not undermine one's reasonable reliance on induction.

(03) Other things being equal, we ought to prefer a theory of modality the acceptance of which does not undermine one's reasonable reliance on induction.

Other things being equal, we ought to prefer other theories of modality to Lewis's modal realism.

Lewis grants (02) and does not dispute (03), so we can assume that the success of the Objection from Scepticism will depend on the strength of the case that can be made for (O1).6

As Lewis understands it, any argument for (O1) will be based primarily on his admission that we have counterparts who are deceived in their reliance on induction. It is possible to detect a certain mischievous relish in his description of our deceived counterparts and the worlds they inhabit:

Some of our deceived counterparts expect the future to resemble the past in the appropriate ways; but they live in worlds where the future does not at all resemble the past. Such worlds exist by recombination: graft any future onto any past .

Some are deceived not about the future but about the past: They live in brand-new worlds full of false traces and records of a past that never was. There might have been a Falsifier to make the false traces. But there needn't have been-for any possible state, there are worlds that begin in just that state.

Some are deceived even about their present . . . unfortunately, their worlds are full of epiphenomenal rubbish that does not interact in any way with them or with anything they can observe. Some are the playthings of powerful field linguists, who irradiate their surfaces so as to prompt assent to falsehoods. Some are brains in vats. However reasonably they theorize, their theories are almost entirely wrong. [Lewis, 1986, p. 116]

His characterization of the argument for (O1), on the basis of his admission that we have these deceived counterparts, is as follows:

Shouldn't the sad fate of all these counterparts and duplicates of ours be a waming to us? What business have we to trust what we call 'reasonable' methods of forming beliefs and expectations, if we know how those methods betray so many others so like ourselves? Why should we expect better luck than theirs? A modal realist has no right to trust induction-he should tum sceptic forthwith. [Lewis, 1986, p. 116]



MODAL REALISMAND INDUCTIVE SCEPTICISM 335

IV. Lewis's Response to the Objection

Lewis thinks that the Objection from Scepticism can be dismissed easily and without much inquiry into details:

The reason everyone has [for mistrusting induction] is that induction is fallible. It is possible, and it is possible in very many ways, that by being reasonable we shall be led into error .... All that, I say, is quite independent of any theory of the nature of possibilities. I recognize the possibilities of error that everyone else recognizes; they are no more and no less possibilities of error for being understood as other worlds of a kind with our own. They give me no more and no less reason to forsake inductive reason than they give to someone who holds a rival metaphysical view of their nature, or to someone who holds no particular view. [Lewis, 1986, p. 117]

He takes this reason for mistrusting induction, which he insists to be the same as everyone else's, to be insufficient. In the following passage, he explains why it might appear that accepting modal realism gives one more reason to mistrust induction than everyone else has:

Outside the philosophy room, it is inevitable that we give our trust to a fallible method. But underneath, we really do not like to do such a thing ...if that is our condition, our discontent is always ready to come alive again if the same old thing-namely that there are abundant possibilities of error-can be said in a new and different way. And one way to give it a new and different look is to tie it to a controversial account of the metaphysical nature of the possibilities in question. That can make it come back to haunt us as vigorously as if it were brand new. Modal realism creates no new reason for scepticism, but it revivifies the old reason. [Lewis, 1986, pp. 117-18]

This is where I take Lewis to be suggesting that the Objection from Scepticism must implicitly involve what I have called 'a Hume Argument', for this is a natural way of understanding his claim that it rests on mistaking "the old reason for scepticism" for a new reason, created by and peculiar to modal realism. This suggestion is reinforced by his claim that "the old reason" really comes to no more than the fact that induction is fallible, since one popular diagnosis of at least some Hume Arguments has it that they are implicitly based on the mistaken assumption that only infallible methods of inference are reasonable.7

Let us stipulate that E is the set of "possibilities of error" recognized by Lewis, and that it includes the sorts of very radical possibilities listed above. This allows us to recast Lewis's crucial claims as an argument that (G1) is false:

(R1) Everyone must recognize all of the possibilities in E.

(R2) Having to recognize all of the possibilities in E gives everyone, whatever his views as to the nature of these possibilities, the same reason to mistrust induction.



336 NOUS

(R3) Having to recognize all of the possibilities in E gives everyone, whatever his views as to the nature of these possibilities, insufficient reason to mistrust induction.

No one-including, of course, the modal realist-has sufficient reason, or any more reason than anyone else has, to mistrust induction solely on the basis of his having to recognize all of the possibilities in E, combined with his views as to the nature of possibilities.

The crucial question is: What is it to recognize a possibility or set of possibilities? A rational, well-informed person whose life depends on his undergoing a tricky and dangerous operation recognizes the possibility that the operation itself might kill him; Lewis apparently recognizes the possibilities that he is being deceived by an evil demon and that the law of gravity will cease to hold five minutes from now, and these two ways of recognizing possibilities seem rather different.

When the patient recognizes the possibility that the operation might kill him, he does more than just acknowledge that this is a possibility-one item on the list of possibilities, along with all of the other possibilities of which he can or can't conceive-that is compatible with all of his experience. He may think that there are many possibilities that are compatible with all of his experience that he does not recognize in the same way. In fact, he takes the possibility that the operation might kill him to be on a select list of possibilities in which he has some substantial reason to believe. In recognizing the possibility that he is being deceived by an evil demon, Lewis may or may not intend to admit that he has some reason to believe that the deceptive demon actually exists.

Let us stipulate that merely to grant the existence of a possibility as compatible with all of one's experience so far is to acknowledge that possibility, while to recognize a possibility as something in which one has some reason to believe is to give credence to it. Does Lewis think that recognizing a possibility is the same as giving credence to it, or merely the same as acknowledging it?

If he takes recognizing a possibility to be the same as merely acknowledging it, his argument can be reformulated as follows:

(R'1) Everyone must acknowledge all of the possibilities in E.

(R'2) Having to acknowledge all of the possibilities in E gives everyone, whatever his views as to the nature of these possibilities, the same reason to mistrust induction.

(R'3) Having to acknowledge all of the possibilities in E gives everyone, whatever his views as to the nature of these possibilities, insufficient reason to mistrust induction.



MODAL REALISM AND INDUCTIVE SCEPTICISM 337

No one-including, of course, the modal realist-has sufficient reason, or any more reason than anyone else has, to mistrust induction solely on the basis of his having to acknowledge all of the possibilities in E, combined with his views as to the nature of these possibilities.

If this is the way in which Lewis intends to argue, then (R1') may be accepted as relatively unproblematic. In insisting on (R2') and (Rj'), however, surely he must simply be assuming something like (R4):

(R4) By itself, acknowledging a possibility gives everyone, whatever his views as to the nature of possibilities, the same, insufficient, reason to give credence to that possibility.

In defending the Objection from Scepticism, I reject this claim, and instead insist on (04):

(04) Acknowledging a possibility gives the modal realist some powerful and distinctive reason, in virtue of his views as to the nature of possibilities, to give credence to that possibility.

Having to give credence to possibilities of error that other philosophers merely acknowledge would surely constitute a special sceptical problem for the modal realist. The truth of (04) may not yet be apparent, but Lewis is surely premature in regarding his response to the objection as decisive before this claim has been clearly articulated and the grounds for and against it have been carefully weighed. Articulating and defending (04) will be the primary purpose of the next section.

While (R"1) - (R"3) seem to be the most natural interpretation of (R1)-(R3), it is possible to formulate another interpretation by reading 'recognize' as 'give credence to':

(R"'1) Everyone must give credence to all of the possibilities in E.

(R"2) Having to give credence to all of the possibilities in E gives everyone, whatever his views as to the nature of these possibilities, the same reason to mistrust induction.

(R"3) Having to give credence to all of the possibilities in E gives everyone, whatever his views as to the nature of these possibilities, insufficient reason to mistrust induction.

No one-including, of course, the modal realist-has sufficient reason, or any more reason than anyone else has, to mistrust induc-



338 NOUS

tion solely on the basis of his having to give credence to all of the possibilities in E combined with his views as to the nature of these possibilities.

The principle (R"2) says that everyone who must give credence to the possibilities in E has, in virtue of his doing so, the same reason to mistrust induction as anyone else in the same position. But giving credence to a possibility is more than just acknowledging it. (R"1) is the strong claim that everyone must give credence to all of the possibilities in E, and the admission that everyone must ackilowledge these possibilities doesn't imply this, so (R"1) must be considered questionable if not downright implausible. Because of the radical nature, the number, and the variety of the possibilities of error in E, giving credence to these possibilities, as opposed to merely acknowledging them, seems a potentially strong reason for mistrusting induction. So (R"3) is also questionable if not implausible.

Since both (R'1)-(R'3) and (R'1)-(R"3) include one or more questionable premises, neither argument provides Lewis with a satisfactory response to the objection. What is more interesting, however, is that not clearly distinguishing between (R'1)-(R'3) and (R"1)-(R"3) can foster the illusion that Lewis has proved that the Objection from Scepticism must implicitly rely on a Hume Argument.

I accept (R'1), and this means that if I were to accept (R'2), I would have to accept (R'3) as well. Given my acceptance of (R'1), if I were to admit that acknowledging the possibilities in E gives everyone the same reason to mistrust induction then, on pain of scepticism, I would have to deny that doing so was a sufficient reason for mistrusting induction.

I also accept (R"2), and this means that if I were to accept (R"1), I would have to accept (R"3), for the much the same reason as above. Given my acceptance of (R"2), if I were to admit that everyone had to give credence to all of the possibilities in E, then, on pain of scepticism, I would have deny that doing so was a sufficient reason for mistrusting induction.

If the different readings of the original (R1)-(R3) are not carefully distinguished from one another, then when the recognition that both (R'1) and (R"2) are plausible is combined with a realization that, on pain of inductive scepticism, (R'3) must be accepted if (R'1) is accepted and (R"3) must be accepted if (R"2) is accepted, one can be left with the impression that anyone who presses the Objection from Scepticism opens the way, not merely to an objection specific to modal realism, but to a Hume Argument. This impression is mistaken. Although I accept (R'1) and (R"2), I deny (R'2), (R'3), (R"'1) and (R"3).

Once doubt has been cast on Lewis's right to claim, a priori, that we are all in the same epistemic boat together, looking at the details of the Objection from Scepticism takes on some real interest.8 Suppose that there is a persuasive argument that Lewis cannot escape inductive scepticism. The crucial question will then be: Does this argument depend only on premises that are equally plausible regardless of one's theory of modality? If so, it is indeed merely a Hume argument. If it depends, however, on one or more premises the acceptance




of which is unavoidable only for the modal realist, then it constitutes a real objection to the theory, and we can leave Lewis alone in his epistemic boat by rejecting modal realism together with these premises. My purpose in the next two sections will be to give a detailed formulation of the argument that Lewis cannot escape inductive scepticism.9

V. The Full Formulation of the Objection From Scepticism

My evaluation of Lewis's response to the objection suggests that the latter will turn out to depend on something like the distinction between giving credence to a possibility and merely acknowledging it. This distinction was rough; my full formulation of the objection will require something in the same spirit but more precise. As a preliminary, it will be helpful to consider the following example:

Suppose Huey believes that there are two cookie jars, A and B, that share a set, Q, of qualitative, observable properties that are collectively possessed by no object besides A or B. We can think of Q as including such properties as being made of earthenware, being shaped like a teddy bear, being 14 inches tall, and so on.

Let us say that this belief is reasonable for Huey and that he holds no conflicting beliefs. Suppose that he is presently observing (and gloating over) a cookie jar that he has stolen, and that Q, the set of properties he believes to be shared by A and B, is also the set of exactly those properties he observes the stolen jar to have. Suppose, moreover, that his present observations, together with his beliefs about A and B, are all of the evidence he has as to the identity and unobserved properties of the stolen jar. I wish to make the following three claims: (1) Huey has conclusive reason to believe that either the stolen jar is A or that it is B; which is to say that given the other things he (reasonably) believes together with his present observations, it is reasonable for him to believe that either the stolen jar is A or it is B, and unreasonable for him to believe otherwise; (2) Huey has some reason to believe that the stolen jar is A, and some reason to believe that it is B; and (3) his reason for believing the stolen jar to be A is exactly as strong as his reason for believing it to be B-neither stronger nor weaker.

I take (3) to imply that, for any respect in which Huey takes A and B to differ from one another, he has no reason whatsoever to believe that the stolen jar has the properties of A rather than those of B. Suppose, for example, that he believes A to have the property containing chocolate chip cookies, and B to have the property containing oatmeal cookies. Neither of these properties is a member of Q, and neither of them is among the properties Huey observes the stolen jar to have-we may imagine that the stolen jar is tightly sealed, although Huey, armed with a hammer, is preparing to remove this obstacle to the satisfaction of his desires. I take (3) to imply that Huey's reason for believing the stolen jar to contain chocolate chip cookies is exactly as strong as his reason for believing it to contain oatmeal cookies, neither stronger nor weaker.

With this illustration in mind, a couple of definitions will allow me to give a precise formulation to my version of the Objection from Scepticism:



340 NOUS

D1: The set 0 is the total set of empirical counterparts of an object x for a person P if and only if:

(i) Each of the members of 0 has all of the (qualitative) properties that x is observed by P to have; and

(ii) No other object has all of these properties.

D2: A set E is the total set of epistemically equivalent alternatives to an object x for a person P if and only if E is the most inclusive set meeting the following conditions:

(i) P has conclusive reason to believe that x is a member of E; and

(ii) For any two members of E, ml and M2, P's reasons for believing x to be identical with ml are exactly as strong as his reasons to believe x to be identical with m2, neither stronger nor weaker. This should be taken to imply that, for any qualitative respect in which ml and m2 differ from one another, P has absolutely no reason to believe that x has the properties of ml rather than those of m2.

In the above example, I claimed in effect that Huey recognizes {A, B } as his own total set of empirical counterparts to his stolen cookie jar, and argued from this that reason compels him to recognize {A, B } as his own total set of epistemically equivalent alternatives to the stolen jar. This particular case is intended to lend plausibility to the general principle, (05):

(05) If a person P satisfies the following conditions:

(i) He reasonably believes that a set E is the total set of empirical counterparts, for him, of some object x;

(ii) He has no conflicting beliefs; and

(iii) He has no evidence as to the identity and characteristics of x apart from: (a) his observation that x has the properties he takes to be shared by the members of E; and (b) his belief that E is the total set of empirical counterparts to x;

then he has conclusive reason to recognize E as the total set of epistemically equivalent alternatives, for him, to x.

The principle (05) is the cornerstone of my version of the Objection from Scepticism. Let us stipulate that W is the set of all the worlds recognized by




Lewis as having exactly those properties he observes the actual world to have. He is in a position relative to W and the actual world that is analogous to Huey's position relative to the set {A ,B I and his stolen jar:

(06) Lewis believes W to be the total set of empirical counterparts, for him, to the actual world. 10

(07) His observation of the properties of the actual world and the belief described in (06) provide him with all the evidence he has as to the identity and characteristics of the actual world.

Together, (05), (06), and (07) imply (08):

(08) Lewis has conclusive reason to recognize W as his total set of epistemically equivalent alternatives to the actual world.

This doesn't quite get us the Objection from Scepticism. We also need the following principle:

(09) Anyone who has conclusive reason to recognize W as his total set of epistemically equivalent alternatives to the actual world cannot reasonably rely on induction.

The principle (09) is plausible in light of the fact that W includes all of those "possibilities of error" that are compatible, by the Recombination Principle, with Lewis's experience of the actual world. Together, (08) and (09) imply the conclusion:

Lewis cannot reasonably rely on induction.

If I can establish that this argument depends on a feature or features peculiar to Lewis's modal realism, then I will have established (s1) without threatening the acceptability Of (02).

Recognizing W as one's total set of epistemically equivalent alternatives to the actual world is a different and worse epistemic position from that of merely recognizing each of the members of W as both possible (in some sense or other) and compatible with one's evidence. (08), in fact, is just a more precise version of (04): the claim that Lewis, as a modal realist, must give credence to possibilities that others need only acknowledge.

Lewis himself can hardly deny that the conditions that are crucial for the application of (05) are satisfied. Since he is in effect committed to (06), he is hardly going to concede either that this commitment is unreasonable or that he has any conflicting beliefs. Furthermore, it is hard to see how he could claim to have any evidence as to the identity and character of the actual world besides his



342 NOUS

observation of its having all the qualitative, observable properties that it shares with the rest of the worlds in W. We will take these considerations to rule out the possibility of his avoiding the objection except by rejecting either (05) or (09). Of these two strategies, rejecting (05) appears to be the more plausible. For the present, then, I will interpret Lewis on the assumption that he grants (09) and rejects (05). This assumption appears to be confirmed by the following passage:

A different conjecture [as to why the Objection from Scepticism seems persuasive] is that ... we are being influenced by the thought that everything is actual. That means that if I am right that there are other worlds, they are not the alternative possibilities I say they are; so what they present is not the mere possibility of error. Instead, the other worlds are more of actuality, and whatever inductive deception occurs therein occurs actually. But if we knew that many people were actually being deceived, that would not be just the old reason for scepticism: distrust induction because it is fallible. No; that would be a good inductive reason to distrust induction. It's not that we should trust induction while it teaches us that it is not to be trusted; rather, our attempt to trust it would destroy itself....

I claim once again that I am within my rights to call the other worlds possible, not actual. If so, they give us no cases of actual inductive error, so they give us no inductive reason to distrust induction. II [Lewis, 1986, p. 1 18]

When Lewis conjectures that the Objection from Scepticism depends on the mistaken assumption that his worlds are "more of actuality", we can take him to be suggesting that (05) ought to be rejected, since such a principle is acceptable only on the understanding that E is taken by P to include nothing but actual objects. Lewis, of course, insists that all but one of the members of W are merely possible.

Whether Lewis's worlds are "more of actuality" may be worth disputing, but it is irrelevant to the objection as I understand it. What is crucial instead is the idea that a person's beliefs about other things of a type with an object with which he is acquainted can be relevant to what it is reasonable for him to believe about that object.

One of the key intuitions underlying modal realism is the idea that a possibility is an alternative to what is actual. Lewis explicitly endorses this idea when he writes, "Possibilities are not parts of actuality, they are alternatives to it." [Lewis, 1986, p. 99] For many purposes, "actuality" can be identified with the entire actual world. The fact that Lewis identifies possibilities with entire concrete worlds, not different in nature from the actual world, suggests that for him, possibilities are alternatives to actuality in the sense of being other things of the same type with it.

Other realist theories do not take possible worlds to be alternatives to the actual world in quite the same way. On an abstract realist theory, possible worlds may be taken to represent alternatives to actuality, where this means that they are taken to be alternatives to a correct representation of the actual world. An abstract realist theory may, for example, identify possible worlds with sets of propositions.




Any set representing the actual world contains nothing but true propositions; sets representing merely possible worlds contain some false propositions. But any set of propositions is the same type of thing as the set representing the actual world, hence is an alternative to the abstract "actual world" in much the same way that Lewis's concrete possible worlds are alternatives to the concrete actual world. 12

In a slightly different vein, someone like Robert Stalnaker can be interpreted as saying that what is actual isn't primarily the world considered as a concrete object, but rather the way the world is-something that is again naturally regarded as an abstract object, this time a "state of affairs", which I take to be a highly complex property. [See Stalnaker, 1976, pp. 65-75.] Merely possible worlds are to be identified with other complex properties of the same type with the way the world is, but differing from it both in various qualitative ways and in not being possessed by anything concrete. We may suppose, for example, that the way the world is includes the property containing someone qualitatively like Napoleon. To say, on Stalnaker's view, that it is possible that no one exactly like Napoleon should ever have existed is to say that there is another complex property-a way a world could be, the same kind of thing as the way the world is-that is not possessed by anything concrete, and which does not include the particular property, containing someone qualitatively like Napoleon.

The crucial difference between Lewis's theory and its rivals (where these include but are not limited to other forms of modal realism), is that Lewis accepts and others do not accept the existence of other things of a type with the concrete actual world-the object that provides us with all of empirical evidence we have, and about which we are mainly concerned to form beliefs by means of induction. The concreteness of Lewis's worlds is relevant to the Objection from Scepticism, not because it forces us to regard them as actual, but because it qualifies them as other things of a type with the actual world. If we were concerned to form beliefs by means of induction about an abstract object, our beliefs about other abstract objects of the same type would be relevant to our justified beliefs about it in just same way that Lewis's beliefs about other concrete worlds are relevant to his justified beliefs about the actual world. This is why the Objection from Scepticism presents a problem that is specific to Lewis's theory, without having to depend on the disputed claim that Lewis's worlds represent "more of actuality".

There is still something seductive about Lewis's claim to be "within his rights to call the other worlds possible . . .", for it seems that merely admitting the existence of empirical counterparts which are or may be merely possible does not-or, more properly, should not-put one in the same epistemic boat with someone who admits the existence of a corresponding set of actual empirical counterparts. My willingness to admit, for example, that in some sense of 'possible', my computer has a possible empirical counterpart that turns into a dragon in five minutes doesn't, by itself, commit me to counting any future dragons as members of my own total set of epistemically equivalent alternatives to my computer.



344 NOUS

Whether it is legitimate or not, however, Lewis's insistence on the (mere) possibility of his worlds is a red herring. We must not allow ourselves to be misled by the fact that Lewis and his opponent may agree on the truth of such sentences as, "There are (merely) possible talking donkeys," and, "There are no actual talking donkeys." Lewis's understanding of the truth-conditions of such sentences is radically different from that of his opponent; for example, in accepting the first, he admits the existence of talking donkeys, while his opponent, in accepting the same sentence, does not (not even when his "quantifiers are wide open").'3

An analogy will serve to illustrate why this difference is so important. Suppose that, while attending an exhibition of gemstones, I point to a case and say, "That case contains twelve diamonds." There are certain consequences that this assertion forces me to accept (given that I have a minimal knowledge of diamonds and the world), such as that the case contains twelve objects made primarily of carbon, that its contents are valuable, that the objects in the case will cut glass, and so on. Now suppose that someone else points to the case and says, "That case contains twelvefake diamonds." He is not agreeing with me, and at the same time pointing out that the diamonds in the case have the interesting property of being fake. He means (presumably) to deny that there are any diamonds in the case; he has certainly not committed himself to the existence of any. Thus it is not surprising that, even if he and I have the same knowledge of diamonds and the world, he can resist many of the consequences about the contents of the case that I must accept.

As long as we reject Lewis's modal realism, there is a compelling explanation of the fact that merely granting the existence of a possible object doesn't have the same epistemic consequences as admitting its actual existence. To say that we "grant the existence" of a possible object is in fact only a manner of speaking; doing so is not tantamount to granting the existence of the object and adding that it has the interesting property of being possible. The difference between granting the existence of an object and granting the existence of a possible object is like the difference between saying that there are diamonds in the case and saying that there are fake diamonds. One can "grant the existence" of a possible object while avoiding the sorts of consequences that would follow from admitting its actual existence, simply because the latter admission does, while the former does not, commit one to the existence of the object. The difference between granting and not granting an object's existence-hardly a trivial matter!-explains why the difference between granting an objects actual existence and (merely) granting its possible existence is important.

When Lewis grants the existence of a possible object, however, he is thereby committed to the existence of that object. His position is analogous to that of a person who thinks that fake diamonds are a special sort of diamond. Such a person is not entitle to say that the objects in the case are fake diamonds and at the same time to resist the obvious consequences of admitting them to be diamonds, not even if he adds, "After all, I only admitted them to be fake




diamonds." His idiosyncratic understanding of the term 'fake' is one that doesn't immunize him from these consequences, as a normal understanding of the term would. Similarly, the term 'possible', even if we suppose for the sake of argument that Lewis is entitled to apply it to his worlds, is not a magical talisman that immunizes him from the consequences of granting the existence of deceptive empirical counterparts of the actual world.

It is true that, on many theories of modality other than Lewis's, "granting the existence" of possible objects commits one to the existence of something; e.g. to the existence of sets of propositions, or of complex, uninstantiated properties.14 Here is the crucial question: Does granting the existence of these sorts of things force us to bear the same epistemic burdens as would our granting the existence of the corresponding objects? If it did, we would be back at square one, with no response to Lewis's insistence that, as far as induction is concerned, he is no worse off than is anyone else. But why should granting the existence of sets of propositions, or uninstantiated properties, or whatever, have the same epistemic consequences as granting the existence of the corresponding objects? The burden of proof falls on the philosopher who wants to claim that it does have the same consequences, or so it seems to me. I see no reason, moreover, to think that the answer to this question should be entirely independent of one's theory of the nature of the entities whose existence constitutes the "existence" of (merely) possible objects.

The claim under discussion may be formulated as follows:

(R5) Regardless of one's theory of possibility, merely granting the existence of a possible object has precisely the same epistemic consequences as granting the existence of the corresponding object.

I reject (R5); everything Lewis says shows that he must accept it.15 I suspect that the (implicit or explicit) acceptance of (R5) (or of a similar assumption) is a primary culprit in generating at least some versions of the problem of induction: If it is true, then inductive scepticism is in fact rationally unavoidable. I don't know how to prove that it is false. But at this point I believe it is sufficient to say that, like nearly everyone else (Lewis being one of the rare exceptions), I am entitled simply to assume that our reliance on induction is rational. This may sound a lot like Lewis's stand in response to the Objection from Scepticism. The difference is that Lewis backs his position with nothing more than a magical invocation of his right to use the expression 'merely possible'; I back mine with a reason, based on the difference between asserting and not asserting the existence of something, why merely granting the existence of a possible object shouldn't (except on Lewis's theory) put us in the same epistemic boat as granting its actual existence.

Although I don't know how to prove that (R5) is false, a case can be made for my claim that if (R5) is true, then inductive scepticism is rationally unavoidable. The argument is simply this: If (R5) is true, then if inductive scepticism is



346 NOUS

unavoidable on Lewis's theory, it is unavoidable on anyone's theory. In the next section I provide further support for the claim that inductive scepticism is unavoidable on Lewis's theory.

VI. Modal Realism and the Reliability of Induction

In the previous section, I assumed that Lewis intended to reject (05):

(05) If a person P satisfies the following conditions:

(i) he reasonably believes that a set E is the total set of empirical counterparts, for him, of some object x;

(ii) he has no conflicting beliefs; and

(iii) he has no evidence as to the identity and characteristics of x apart from: (a) his observation that x has the properties he takes to be shared by the members of E; and (b) his belief that E is the total set of empirical counterparts to x;

then he has conclusive reason to recognize E as the total set of epistemically equivalent alternatives, for him, to x.

It is possible, however, to read Lewis as granting (05), in which case, in defending his claim that the modal realist has no special reason for mistrusting induction, he will almost certainly want to insist that it follows from the following more general principle:

(R6) If a person P satisfies the following conditions:

(i) he reasonably believes that a set E is the total set of actual or possible empirical counterparts, for him, of some object x;

(ii) he has no conflicting beliefs; and

(iii) he has no evidence as to the identity and characteristics of x apart from: (a) his observation that x has the properties he takes to be shared by the members of E; and (b) his belief that E is the total set of empirical counterparts to x;

then he has conclusive reason to recognize E as the total set of epistemically equivalent alternatives, for him, to x.16

The principle (R6) implies that no special Objection from Scepticism can be formulated against modal realism, but (R6) will be acceptable only if (09),is rejected:




(09) Anyone who has clear and decisive reason to recognize W as his total set of epistemically equivalent alternatives to the actual world cannot reasonably rely on induction.

If Lewis were to accept both (R6) and (09), he would be saying in effect that the Objection from Scepticism is powerless against his theory only because no one can reasonably rely on induction. Such a position is not one we would be likely to take seriously; nor does it sound much like what Lewis intends to be saying. For the remainder of this section, I will interpret Lewis's responses to the Objection from Scepticism on the assumption that he accepts (R6) and is attempting to reject (09). I argue that he is not entitled to reject (09).

In admitting W to be his total set of epistemically equivalent alternatives to the actual world, Lewis admits that his reason for believing the actual world to be any one of the members of W is exactly as strong as his reason for believing it to be any of the others. This means, for example, that he takes his reason for believing the actual world to be any one of the worlds in W that is inhabited by an omnipotent deceptive demon to be exactly as strong as his reason for believing it to be any one of these worlds that obeys a tidy set of natural laws. It isn't obvious that reason alone compels such an admission; and since the number and variety of deceptive worlds in W is very large, the view that it might undermine one's reasonable reliance on induction is at least plausible.

There are, however, accounts of what it is for a method of inference to be reasonable that fit nicely with a rejection of (09), and Lewis hints at one such account in the following passage:

Outside the philosophy room, it is inevitable that we give our trust to a fallible method. But underneath, we really do not like to do such a thing. And calling the fallible method 'inductive reason'-as we are right to do, because that is indeed the name we have given it-does not make us like it any better. [Lewis, 1986, p. 117, my emphasis]

This suggests that the reasonableness of accepted methods of induction is a matter of convention: Induction is reasonable simply because we call it 'reasonable' and use it with approval. If this were correct, Lewis's acceptance of W as his total set of epistemically equivalent alternatives to the actual world would have no power to threaten the reasonableness of his inductive inferences.

Theories like that suggested in the passage above have been defended by such philosophers as P. F. Strawson and Paul Edwards. A powerful critique of such theories is offered by Simon Blackburn. He grants that such a theory answers the question of what makes induction reasonable, but points out that it raises another question: Why should we have any confidence in the conclusions of reasonable inferences? He thinks that the latter question is incoherent:

...if one thing is clear in the normal use of the word "reason", it is that once you have really shown that you have a reason for a belief, you need do nothing more to justify taking the reason as relevant to the amount of confidence to have in that be-



348 NOUS

lief. We often discuss whether evidence of some sort really is a reason for a belief, but once that question is settled, there is no further question whether this reason is relevant to the amount of confidence which we can justifiably place in the belief. [Blackburn, 1973, p. 21]

Now confidence, in the present context, is not a purely psychological matter. According to Blackburn, there is a point to reasoning: We use it because we are trying, as far as possible, to have confidence in beliefs that are true. Thus in order for the ways in which we form beliefs to count as reasonable, there must be some real or putative tendency for beliefs that are formed in those ways to be true.

In other words, there must be some connection between a method's being reasonable and its being reliable. But many philosophical problems arise at this point, some of which have to do with explaining what it takes to qualify a method as reliable. It would clearly be too stringent to require reliable methods of inference always or necessarily to yield beliefs that are true, but it isn't clear what is required. Is it sufficient that, as a matter of fact, following the method results in true beliefs a high enough proportion of the time? And what proportion is high enough? Is it also a requirement that a method's yielding true beliefs is not, as it were, a mere accident?

Even if we set aside the problems with the notion of reliability, the connection between a method's being reliable and its being reasonable is far from straightforward. We cannot simply say that a method of inference is to be counted as reasonable if and only if it is reliable. For one thing, it might be perfectly reasonable for someone to use a method of inference that was not at all reliable, as long as he reasonably took it to be so. For another thing, the reliability of method of inference might be neither believed nor suspected by anyone, and in fact might be something for which no one had the slightest evidence, in which case its actually being reliable wouldn't seem to be enough, all by itself, to imply that it was a reasonable method for anyone to use. Thus a method's actually being reliable appears to be neither a necessary nor a sufficient condition for its use to be reasonable.

The trouble with the Strawson-Edwards account of reason, however, is that it represents a method's being reasonable as having nothing to do with its having any tendency, whether real or imagined, to result in true beliefs, and this flies in the face of our recognition that the whole point of reasoning is to acquire true beliefs. If Blackburn is right, there is a kind of pragmatic inconsistency in having or trying to have confidence in a method of inference one knows perfectly well to be unreliable-sat least insofar as one is trying to be reasonable. Without giving a full account of the connection between a method's being reasonable and its being reliable, perhaps we can at least accept the following principle:

(O10) If someone has a clear and conclusive reason for believing that a method of inference is unreliable, then his reliance on that method of inference is unreasonable.




The principle (10) supports the Objection from Scepticism when it is conjoined with (01 1):

(11) Recognizing W as one's total set of epistemically equivalent alternatives to the actual world is a clear and conclusive reason for believing induction to be unreliable.

Together, (10) and (O1 1) imply (09). In rejecting (09) and claiming that modal realism gives him "no more and no less reason to forsake inductive reason than everyone already has", Lewis must therefore be rejecting either (010) or (O 1).

There are two reasons for attributing a rejection of (O II) rather than of (oIo) to Lewis, in spite of the fact that he seems to be flirting with some form of conventionalism in the passage quoted above. First, (010) is a plausible principle which asserts a relatively uncontroversial connection between a method's being reasonable and its being reliable, and Lewis's position will be weaker insofar as it involves rejecting this principle. Secondly, Lewis discusses a version of the Objection from Scepticism formulated by Peter Forrest, which is in effect an argument that modal realism implies that induction is unreliable [see Forrest, 1982, pp. 456-64]; he responds with an attempt to refute Forrest's argument that modal realism has this implication. Thus Lewis apparently grants (O10), and this means that he must reject (O1 1).

It is interesting to recall the passage quoted on p. 342 above, beginning with, "A different conjecture is that once again we are being influenced by the thought that everything is actual." This might be taken to suggest that a reasonable use of induction depends only on one's not having clear and conclusive reason for thinking it to be actually unreliable. Lewis's acceptance of W as his total set of epistemically equivalent alternatives to the actual world is not an admission that induction is actually unreliable; so perhaps it is not enough to make his reliance on induction unreasonable.

Given the assumptions we are now making, however, this response won't do. W isn't just any old set of possibilities: We are already assuming that Lewis's recognition of W as his total set of epistemically equivalent alternatives to the actual world means that he takes his reason for believing the actual world to be any one of the members of W is exactly as strong as his reason for believing it to be any of the others. He cannot reasonably claim to know or believe that the actual world is any particular member of W, and so his beliefs regarding the reliability of induction in W taken as a whole, and not merely his beliefs regarding its actual reliability, determine whether his use of induction is reasonable.

Consider an analogy: Suppose someone admits the existence of regions in the actual world in which induction is unreliable, perhaps because he thinks there are deceptive demons whose power extends throughout a radius of fifty miles. Such an admission might not, by itself, make it unreasonable for him to rely on induction (although the belief that he was in such a region would presumably do so). Suppose, however, that he recognizes the existence of R, a set of regions of



350 NOUS

the actual world such that his reasons for believing himself to be in any one of these regions are exactly as strong as his reasons for believing himself to be in any of the others. In this case, whether his reliance on induction is reasonable will depend on whether he thinks that induction is unreliable relative to R taken as a whole, which in turn would seem to depend on the proportion of deceptive to undeceptive regions in R. Similarly, it looks as if whether Lewis is reasonable in relying on induction will depend on something like the proportion of deceptive to undeceptive worlds in W.

Although I will not discuss Forrest's argument in any detail, the following response to the Objection from Scepticism can be offered on Lewis's behalf on the basis of his response to Forrest17: Acceptance of W as Lewis's total set of epistemically equivalent alternatives to the actual world is clear and conclusive reason for believing induction to be unreliable only if there is some objective sense in which the deceptive worlds are "more abundant" than the undeceptive worlds in W. There is, however, no objective sense in which the deceptive worlds are more abundant than the undeceptive worlds in W. The cardinality of the set of deceptive worlds is infinite and equal to that of the set of undeceptive worlds. This being so, any argument that the deceptive worlds are more abundant must involve the erroneous assumption that there is some natural order in which the worlds in W must be considered. If there were some natural order in which the worlds in W had to be considered, it would be possible to calculate something like a limiting frequency of the deceptive worlds among the worlds in W, in much the same way that considering the natural numbers from least to greatest permits one to calculate the limiting frequency of the even numbers among the natural numbers (this being .5) or of the prime numbers among the natural numbers (this being 0). There is, however, no other way to make sense of the claim that the deceptive worlds are more abundant. So, since there is in fact no objective order in which the worlds in W must be considered, it follows that there is no objective sense in which the deceptive worlds are more abundant than the undeceptive worlds in W.

As it happens, both of the crucial claims on which the above argument depends are questionable. It isn't clear that the only way to make sense of the claim that the deceptive worlds are more abundant than the undeceptive worlds in W involves assuming that there is an objective order in which these worlds must be considered. It has been suggested (e.g. in Bigelow, 1976, pp. 299-320) that the relative abundance of a type of world in a given set of worlds might be connected with the variety of worlds of that type, and it is plausible to suppose that there is a far greater variety of deceptive worlds in W than of undeceptive worlds. While I won't pursue this suggestion here, it seems worthy of more serious consideration than Lewis gives it.

Suppose, however, that we grant that if the deceptive worlds are more abundant than the undeceptive worlds, this can only be because there is some objective ordering in which the worlds must be considered. Forrest claims (roughly) that for every undeceptive world, there is an infinite number of




empirically indistinguishable deceptive worlds. Although a partial ordering of the worlds into empirically indistinguishable "equivalence classes" might not correspond to any objective relation among the worlds in W, it might still be objectively relevant to the question of what we are justified in believing about the identity and properties of the actual world.

But suppose that we were to accept the whole argument offered on Lewis's behalf. What this argument really establishes is that it is not possible to determine, in a meaningful way, the relative abundance of either the deceptive or the undeceptive worlds among the worlds of W. Assigning any value at all to the relative abundance of deceptive or undeceptive worlds in W implies the existence of an objective ordering in which these worlds must be considered. It would seem, however, that regarding induction as reliable in W involves taking the relative abundance of undeceptive worlds in W to be high enough (however high this might be). Since one cannot coherently regard induction as reliable in W if no value at all can coherently be assigned to the relative abundance of undeceptive worlds in W, the argument attributed to Lewis implies that he cannot coherently regard induction as reliable.

VII. Conclusion

The argument that Lewis cannot escape inductive scepticism depends on the claim that he must accept both (05) and (06). That Lewis must accept (06) follows in a straightforward way from his commitment to modal realism together with (D1), the definition of 'empirical counterpart'; but no one who rejects modal realism must accept this premise. So far, then, I have fulfilled my promise to present a version of the Objection from Scepticism that has its basis in a premise to which Lewis, but not everyone else, is committed.

On the other hand, Lewis claims that we are all in the same epistemic boat together, and his right to make this claim depends on his acceptance of (R5). for the opponents of modal realism who deny that, regardless of one's theory of modality, inductive scepticism is unavoidable, there is therefore a choice between rejecting (05) and rejecting (R5). While I believe that it would be wrong to reject (05), I don't pretend that doing so would be impossible for a strong- minded philosopher. Nonetheless, it seems to me that (05) is plausible and that-apart from acceptance of modal realism-there is no good reason to reject it; and also that (R5) is implausible and that-apart from modal realism-there is no good reason to accept it. If acceptance of (05) and rejection of (R5) is, as I believe, the natural position for Lewis's opponents, then they should agree that the Objection from Scepticism presents Lewis with a serious difficulty.

The claim that one cannot accept Lewis's theory and resist scepticism is noteworthy not merely because it provides the basis for an objection to Lewis's modal realism; it is also interesting because of what it implies for the notion of reason. I assume, in agreement with Lewis, that reliance on induction is reasonable. Explaining why this is so-giving a satisfactory account of the nature of reason and justification as these pertain to our use of and attitudes



352 NOUS

towards induction-is one of the enduring problems of philosophy. The conclusion for which I have argued shows that the reasonableness of one's reliance on induction may be defeated, not merely by one's beliefs as to what is actual, but by one's beliefs as to the nature and variety of possibilities. Although I have insisted that Lewis's problem is not a perfectly general one, perhaps others besides Lewis should take warning, for there may be other theories of modality the acceptance of which would tend to undermine one's reasonable reliance on induction.

Finally, I have simply assumed that if accepting a given theory of modality undermines one's ability reasonably to rely on induction, then this constitutes a reason for rejecting that theory. Supposing that he could be brought to accept the rest of my argument, however, it is possible to imagine Lewis responding as follows: "Modal realism ought to stand or fall solely on the basis of the evidence for it and against competing theories. We cannot reject theories simply because they have consequences we dislike. If our best theory of the solar system implies that the sun is likely to explode tomorrow, we cannot reject the theory just because we find this consequence upsetting. The same thing holds if our best theory of modality implies that reliance on induction is unreasonable."18

While this might seem to be a rather desperate response to the objection, it is worth asking what we commit ourselves to if we reject it. Perhaps we commit ourselves to the view that the reasonableness of induction is itself a piece of evidence-and clearly an important piece-that must be considered in deciding which theories of modality are worthy of belief. I suspect, however, that we commit ourselves to the view that a reasonable reliance on induction involves an a priori justified rejection of any theory of modality the acceptance of which would make such reliance impossible. In this case, accounting for such a priori justified beliefs will be a significant and problematic part of giving an account of reason and justification.

Notes lLewis finds the abstract/concrete distinction to be problematic. Without intending to

imply that his worries about it are unjustified, I will ignore them here, since these issues are not particularly relevant to the substance of my argument.

2He adds that every way that a part of a world could be is a way that a part of a world is. I ignore this for the sake of simplicity.

3Lewis says that this point was brought to his attention by Peter van Inwagen. 4Lewis is also open to the possibility that other worlds contain "alien" properties and

things, i.e. properties and things of a sort that don't occur at all in the actual world. If other worlds contain alien properties and things, then not all of the worlds could be manufactured by copying and pasting together pieces of the actual world. What is crucial for the present, however, is the fact that Lewis is committed to the existence of at least those worlds we can imagine being manufactured in the way described.

5j adopt the following convention for the numbering of principles: Those that play a crucial role in my formulation of the Objection from Scepticism will be numbered (On); those that play a crucial role in a response to the objection that is attributed (perhaps tentatively) to Lewis will be numbered (R.).




6Although Lewis does not dispute (03), it's credentials are worth wondering about. I will have something to say in the conclusion about the commitments that may be involved in accepting this principle.

7For a defense of such an interpretation of Hume's own arguments, see Stove, 1965. For an argument that this is a mistaken interpretation of Hume's arguments, see Stroud, 1977, pp. 42-67. Whether or not Lewis in fact means to say that the objection rests on a Hume Argument, such a claim has been explicitly endorsed in discussion by several philosophers who view themselves as agreeing with the substance of the above passage.

8That is, it takes on more interest, for the present purposes, than it would have if it were known to be based on a Hume argument. I don't mean to say that Hume arguments are unworthy of philosophical interest.

9What has become of Lewis's claim that recognizing the possibilities of error that he recognizes is just recognizing that induction is fallible? I believe that this claim is implausible on either interpretation. Recognizing that induction is fallible, in the way we ordinarily do, seems to involve something more than merely acknowledging all of the possibilities in E, for one might conceivably do this without conceding that there was any reason whatsoever to believe that induction will ever let us down. On the other hand, it seems to involve something less than giving credence to all of the possibilities in E, for while we do admit that induction is fallible, we don't thereby commit ourselves to taking some of the wilder possibilities of error really seriously, in the way that we would presumably be doing if we were to give credence to them.

10(06) is to be understood so as to follow in a straightforward way from Lewis's acceptance of modal realism together with (D1). This doesn't mean that Lewis would be happy with the way (06) is expressed; he would no doubt insist that the empirical counterparts he believes in are merely possible. (I will explain later on in this section why I think this makes no difference.) Perhaps a more precise wan of expressing what (06) is intended to say would be as follows: Lewis believes W to be a set of objects that, by (D1), qualify as empirical counterparts, for him, to the actual world.

I It is worth noting that in this passage, Lewis admits only that knowing the existence of many actual deceived counterparts would be a good inductive reason to mistrust induction. I hold that if one believes in the existence of enough deceived empirical counterparts of oneself, this isn't merely inductive reason to mistrust induction: It (logically) prevents one's trust in induction from being reasonable (though it doesn't logically imply that trusting induction will always result in false beliefs).

121n fact, 'actual' is likely to be an ambiguous term for an abstract realist. By 'the actual world' he may mean the concrete actual world - the very same object that Lewis identifies as the actual world; but he may also mean the privileged object, among all those abstract objects that he identifies with possible worlds, that correctly represents or corresponds to the (concrete) actual world.

13My argument here draws some inspiration from Armstrong, 1989; especially section II of chapter 3.

141t may be possible to identify some such objects as "abstract versions" of the corresponding concrete objects; e.g. a certain set of propositions may be identified as an "abstract talking donkey". In a theory where such identifications are possible, the point of the preceding paragraph may be put by saying that an abstract talking donkey is not a talking donkey.

15As with (06), I don't mean to say that Lewis would be happy with my formulation of (R5); he would no doubt object that, as it stands, we will tend to understand it as having an implicit 'actual' before the last occurrence of the world 'existence', and that on this understanding, he also rejects it. In justice to Lewis, I can only enjoin the reader to avoid reading in the offending occurrence of 'actual'. On the other hand, I certainly don't want (R5) to be understood as having an implicit 'possible' before either the least occurrence of the word 'existence' or the last occurrence of the word 'object'. This would make it trivially true, mainly because, at least for those who reject Lewis's theory, it changes the sense of what is said in a non-trivial way.



354 NOUS

16(R6) is stronger than (R5): The former implies the latter and not vice versa. Lewis is committed to accepting (R6), however, in accepting both (R5) and (05). Thus, if the argument of this section is successful, it shows that everything hangs on (05) - that rejecting this principle is Lewis's only plausible strategy for avoiding the Objection from Scepticism.

r7In no way do I imply that Forrest's argument is unworthy of consideration, but it is a different argument from that in which I am chiefly interested. The rest of this section contains a rather free adaptation of Lewis's response to Forrest as it appears on pp. 118- 21 of Lewis, 1986, under the assumption that Lewis admits W to be his total set of epistemically equivalent alternatives to the actual world. Since I don't know whether Lewis endorses this assumption, I don't know whether he would accept the argument formulated on his behalf.

18j don't mean to say that Lewis would have to respond as I have imagined - only that it would be open to him to do so.

References

Armstrong, David M. 1989 A Combinatorial Theory of Possibility, Cambridge: Cambridge University Press Bigelow, Jonathan 1976 'Possible Worlds Foundations for Probability', Journal of Philosophical Logic, 5 Blackburn, Simon 1973 Reason and Prediction, Cambridge: Cambridge University Press Forrest, Peter 1982 'Occam's Razor and Possible Worlds', Monist 65 Lewis, David 1986 On the Plurality of Worlds, Oxford: Blackwell Stalnaker, Robert 1976 'Possible Worlds', Nous, 10 Stove, D. C. 1965 'Hume, Probability, and Induction', The Philosophical Review, 7 Stroud, Barry 1977 Hume, London: Routledge & Kegan Paul



Modal Realism and Inductive Scepticism

Documents

Transcript of Modal Realism and Inductive Scepticism